Temperature-Induced Mechanomodulation of Interpenetrating Networks of Star Poly(ethylene glycol)-Heparin and Poly(N-isopropylacrylamide).

Thermoresponsive interpenetrating networks (IPNs) were prepared by sequential synthesis of a biohybrid network of star-shaped poly(ethylene glycol) [starPEG] and heparin and a poly(N-isopropylacrylamide)-polymer network. Amide bond formation was used for cross-linking of the starPEG-heparin network and photo-cross-linking with N,N'-methylenebis(acrylamide) was applied for the formation of the second polymer network. Both networks were linked by chain entanglements and hydrogen bonds only. The obtained sequential IPNs (seq-IPNs) showed temperature-dependent network properties as reflected by swelling and elasticity data as well as by the release of glycosaminoglycan-binding growth factors. The elastic modulus of the seq-IPNs was found to be amplified up to 50-fold upon temperature change from 22 to 37 °C compared to the intrinsic elastic moduli of the two combined networks. The heparin concentration (as well as the complexation of growth factors with the hydrogel-contained heparin) was demonstrated to be variably independent from the mechanical properties (elastic moduli) of the hydrogels. Illustrating the usability of the developed seq-IPN platform for cell fate control, the thermo-modulation of the release of vascular endothelial growth factor (VEGF) and bone morphogenetic protein 2 (BMP-2) is shown as well as the osteogenic differentiation of human mesenchymal stem cells exposed to stiff and BMP-2 releasing seq-IPNs.

Polyolefins Formed by Chain Walking Catalysis-A Matter of Branching Density Only?

Recently developed chain walking (CW) catalysis is an elegant approach to produce materials with controllable structure and properties. However, there is still a lack in understanding of how the reaction mechanism influences the macromolecular structures. In this study, a series of dendritic polyethylenes (PE) synthesized by Pd-α-diimine-complex through CW catalysis (CWPE) is investigated by means of theory and experiment. Thereby, the exceptional ability of in situ tailoring polymer structure by varying synthesis parameters was exploited to tune the branching architecture, which allowed us to establish a precise relationship between synthesis, structure, and solution properties. The systematically produced polymers were characterized by state-of-the-art multidetector separation and neutron scattering experiments as well as atomic force microscopy to access molecular properties of CWPE. On a global scale, the CWPE appear in a worm-like conformation independently on the synthesis conditions. However, severe differences in their contraction factors suggested that CWPE differ substantially in topology. These observations were verified by NMR studies that showed that CWPE possess a constant total number of branches but varying branching distribution. Small angle neutron scattering experiments gave access to structural characteristics from global to segmental scale and revealed the unique heterogeneity of CWPE, which is predominantly based on differences in their dendritic side chains. The experimental data were compared to theoretical CW structures modeled with different reaction-to-walking probabilities. Simple theoretical arguments predict a crossover from dendritic to linear topologies yielding a structural range from purely linear to dendritic chain growth. Yet, comparison of theoretical and empirical scattering curves gave the first evidence that a transition state to worm-like topologies is actually experimentally accessible. This crossover regime is characterized by linear global features and dendritic local substructures contrary to randomly hyperbranched systems. Instead, the obtained CWPE systems have characteristics of disordered dendritic bottle brushes and can be adjusted by the walking rate/reaction probability of the catalyst.

Biohybrid networks of selectively desulfated glycosaminoglycans for tunable growth factor delivery.

Sulfation patterns of glycosaminoglycans (GAG) govern the electrostatic complexation of biomolecules and thus allow for modulating the release profiles of growth factors from GAG-based hydrogels. To explore options related to this, selectively desulfated heparin derivatives were prepared, thoroughly characterized, and covalently converted with star-shaped poly(ethylene glycol) into binary polymer networks. The impact of the GAG sulfation pattern on the network characteristics of the obtained hydrogels was theoretically evaluated by mean field methods and experimentally analyzed by rheometry and swelling measurements. Sulfation-dependent differences of reactivity and miscibility of the heparin derivatives were shown to determine network formation. A theory-based design concept for customizing growth factor affinity and physical characteristics was introduced and validated by quantifying the release of fibroblast growth factor 2 from a set of biohybrid gels. The resulting new class of cell-instructive polymer matrices with tunable GAG sulfation will be instrumental for multiple applications in biotechnology and medicine.

A model for segregation of chromatin after replication: segregation of identical flexible chains in solution.

We study the segregation of two long chains from parallel but randomly twisted start conformations under good solvent conditions using Monte Carlo simulations to mimic chromatin segregation after replication in eukaryotic cells in the end of prophase. To measure the segregation process, we consider the center-of-mass separation between the two chains and the average square distance between the monomers which were connected before segregation starts. We argue that segregation is dominated by free diffusion of the chains, assuming that untwisting can be achieved by Rouse-like fluctuations on the length scale of a twisted loop. Using scaling analysis, we find that chain dynamics is in very good agreement with the free diffusion hypothesis, and segregation dynamics follows this scaling nearly. Long chains, however, show retardation effects that can be described by a new (to us) dynamical exponent, which is slightly larger than the dynamical exponent for Rouse-like diffusion. Our results indicate that nearly free diffusion of chains during a timescale of a few Rouse-times can lead to segregation of chains. A main obstacle during segregation by free diffusion is random twists between daughter strands. We have calculated the number of twists formed by the daughter strands in the start conformations, which turns out to be rather low and increases only with the square-root of the chain length.